If you’re reading this, it’s a fair bet that you, like me, are too young to have seen Psycho when it first came out, in 1960. And for anyone who didn’t see it then, it’s probably safe to say that none of us can ever fully know what it felt like to experience the shock — the sheer bloody jaw-dropping terror — of Alfred Hitchcock’s game-changing masterpiece of horror. Imagine the shivery jolt of the opening shark attack in Jaws, magnified 500 times. Because in Jaws, of course, everyone knows, on some level, what’s coming. Hitchcock, by contrast, used Psycho to play the ultimate dirty trick. He killed off his lead actress, Janet Leigh, halfway through the movie, and he did it with such unspeakable out-of-nowhere savagery that he seemed to be pulling the rug, the floor, and the earth right out from under the audience. He opened an abyss, exposing moviegoers to a dark side that few, at the time, could ever have dared to imagine.

Psycho was adapted from a novel that was based on the case of Ed Gein, the demented murderer and graverobber from rural Wisconsin who became the first — and still most legendary — of all modern serial killers. (Twenty-four years later, he inspired The Texas Chainsaw Massacre as well; he was the sick puppy who kept on giving.) But it’s doubtful, in the early ‘60s, that almost any American had ever even heard the term “serial killer.” We were still a long way from Ted Bundy, Charles Manson, or chianti jokes.

In the famous shower scene, when that big, fat kitchen knife, wielded by a mysterious Victorian shrew named Mrs. Bates, came slashing down, over and over again (Skree! Skree! Skree!), into the body of Marion Crane, it was also slicing through years — decades, centuries — of audience expectation that the hero or heroine of a fictional work would be shielded and protected, or would at least die (usually at the end) in a way that made some sort of moral-dramatic sense. In Psycho, murder made no sense at all; the suddenness — and viciousness — of it tore at the fabric of our certainty. What it suggested is that none of us, in the end, are ever truly protected.

You could easily claim that Psycho, more than any other film, is the movie that changed movies — that it broke down, and reconfigured, popular storytelling by shifting it from a form in which lives were orderly and cohesive, bound by the symmetrical conflicts associated with classic Hollywood, to one in which lives were loose, random, unpredictable, and violent, subject to the messiness we associate with the Hollywood films of the ‘70s after the collapse of the studio system.

But the most measurable and seismic effect that Psycho had was on the horror genre itself. Before Psycho, horror movies were “monster” movies. They were fantasies in which men battled supernatural creatures — or turned into them. The monsters could be big (Godzilla) or small (The Fly), sexy (Dracula) or ugly (Frankenstein); they could be spectral and profound (I Walked With a Zombie) or literal and rubbery (The Creature From the Black Lagoon); they could come from outer space (Invasion of the Body Snatchers) or they could be the beast within (Dr. Jekyll & Mr. Hyde). But they were all, one way or another, not quite of this earth.

Psycho revolutionized all that. Here was a horror film in which the “monster” lived inside the head of one man — poor, schmucky Norman Bates, the mamma’s boy with a black secret. In truth, there was no monster at all, no shrieking outsize “mother.” There was just Norman and his rage. Yet Hitchcock’s genius is how deftly he created the illusion of a monster. The Bates house, that looming Victorian mansion full of cryptlike rooms and stuffed birds, was, in effect, a symbol of old-fashioned 19th-century terror. It was a Hollywood funhouse with a secret trap door.

Once inside that house, Hitchcock, drawing his camera back and up, up, up high, teased the audience with a great Freudian metaphor. Though he never, right up until the end, let us get close enough to see Mrs. Bates, what we did see was Norman carrying her around — which, of course, is exactly what the real monsters of our time do. They carry their private demons around, becoming slaves to them instead of mastering them. They become souls in demon drag.

By making the audacious claim that the darkest monsters — brutal, homicidal, and unknowable — live directly inside us, Alfred Hitchcock, in the grandest stunt of movie history, did more than kill off his heroine. He made a show of killing God; he expressed the horror of a world that had seen enough real horror (World War I, the Holocaust, the dropping of the A-bomb) not to need any more monsters. And that’s why the horror films of today are forever in his debt, and in his shadow. Every time you see a slasher movie with Jason, Freddy, Michael Myers, or whatever new name they come up with for some hulk in a mask with a big blade, you’re watching a remake of Psycho — an attempt to recapture its fear and insanity. But, of course, that can never happen again. Because now we know what’s coming. The movies, it turned out, could only kill God once.

For discussion: Is Psycho the scariest movie you’ve ever seen? If not, what is? And why is Psycho still so terrifying no matter how many times you’ve seen it?